diff --git a/pantheon.upstream.yml b/pantheon.upstream.yml
index ee00ad8e7ca..163ab10ac38 100644
--- a/pantheon.upstream.yml
+++ b/pantheon.upstream.yml
@@ -4,3 +4,6 @@
 # For more information see: https://pantheon.io/docs/pantheon-upstream-yml
 api_version: 1
 php_version: 7.2
+
+# See https://pantheon.io/docs/pantheon-yml/#enforce-https--hsts for valid values.
+enforce_https: transitional